Neuropeptides, small signal peptides traditionally believed to function largely as transmitters of nerve impulses in the brain which mediate mood and emotion have not been shown to also regulate immune system function. Monocytes have receptors and will respond chemotactically to numerous neuropeptides. The doses for these reponses are very low, in the 10 to the -14 M range, making this class of compounds among the most potent biological agents described. These results imply the existence of a neuroendocrine link between the brain and the immune system whose purpose is to integrate behavioral and emotional responses with immune system function. Additionally, we have shown that small cell lung cancer has surface antigens considered typical of macrophages. This disease may thus represent a dysfunction in the monocytic component of the immune system and may not be derived from lung cells as current dogma predicts. The neuropeptide bombesin, present in lung tumor cells, is chemotactic and growth promoting for these cells and may be involved in the pathogenesis of cancer as well as inflammatory diseases.